1. Field of the Invention
This invention relates in general to apparatus and a method for monitoring a system operation and more particularly to such apparatus and method for monitoring and visually displaying the distributions of various physical quantities occurring within the core of a nuclear power reactor.
2. Description of the Prior Art
The controlled release of large amounts of energy through nuclear fission is now quite well known. In general, a fissionable atom such as U.sup.233, U.sup.235, or PU.sup.239 absorbs a neutron in its nucleus and undergoes a nuclear disintegration. This produces on the average, two fission products of lower atomic weight and great kinetic energy, several fission neutrons, also of high energy, and fission gamma rays.
The kinetic energy of the fission products is quickly dissipated as heat in the nuclear fuel. If, concurrent with this heat generation, there is at least one neutron remaining which induces a subsequent fission, the fission reaction becomes self-sustaining and the heat generation is continuous. The heat is removed by passing a coolant through heat exchange relationship with the fuel. The reaction may be continued as long as sufficient fissionable material exists in the fuel to override the effects of fission products and other neutron absorbers which may also be present and of neutron leakage from the active region of the core.
In order to maintain such fission reactions at a rate sufficient to generate useful quantities of thermal energy, nuclear reactors are presently being designed, constructed and operated in which the fissionable material or nuclear fuel is contained in fuel elements which may have various shapes, such as plates, tubes or rods. These fuel elements are usually provided on their external surfaces with a corrosion resistant, non-reactive cladding which contains no fissionable material. The fuel elements are grouped together at fixed distances from each other in a coolant flow channel or region as a fuel assembly, and a sufficient number of fuel assemblies are arranged in a spaced array to form the nuclear reactor core capable of the self-sustained fission reaction referred to above. The core is usually enclosed within a reactor vessel.
Commonly, in pressurized water reactors, a neutron absorbing element is included within the cooling medium (which also functions as a moderator) in controlled variable concentrations to modify the reactivity, and thus the rate of heat generation within the core, when required. In addition, control rods are interspersed among the fuel assemblies, longitudinally movable axially within the core, to control the core's reactivity and thus its power output.
Control of the nuclear power distribuion within the core is a primary consideration in nuclear reactor operation. A mal-distribution may be detected by analysis of data from in-core and/or ex-core instrumentation. One conventional technique for monitoring the core power distribution involves the monitoring of the neutron flux distribution or the fission gamma ray distribution at a large number of locations within the core with the aid of fixed in-core detectors. While this approach is reliable and effective in satisfying its intended purpose, it tends to be relatively expensive to implement and so it is not universally followed. The alternative technique relies in the main on the use of a small number of sets of comparatively inexpensive ex-core neutron detectors arranged around the outside of the reactor vessel. In pressurized water reactor (PWR) installations in which this latter, alternative approach is used to insure the safety of reactor operations, an in-core instrumentation system that relies on periodically passing small neutron detectors through a number of instrumentation thimbles that are expressly provided for that purpose in the core is commonly utilized. This system is used on a periodic basis, normally about once each full power month, to collect information on which recalibration of the fixed ex-core system is based and to provide detailed reference measured power distribution information for verification of compliance with the plant Technical Specifications that regulate operation. One compromise between these two basic approaches involves the installation in the reactor of a few strings of fixed in-core neutron or gamma ray detectors. Virtually all of the commercial PWR units that rely primarily on ex-core neutron detectors for continuous power distribution monitoring have provisions for the installation of up to eight strings of fixed in-core gamma ray detectors in preselected locations. Installation of these strings of in-core detectors in the preselected locations does not inhibit operation of the movable in-core detector system and so can be accomplished at modest expense and inconvenience. Within the scope of this disclosure, the use of exclusively ex-core neutron detectors and of a few strings of fixed in-core detectors is considered synonomous: both are to be distinguished from the use of large numbers of strings of fixed in-core detectors.
The methodologies both for ex-core neutron detector recalibration and for synthesis of a detailed three dimensional core power distribution from movable detector traces are well known in the art and are in common use. Since the in-core movable detector system is actually used for core power distribution monitoring only infrequently, and usually only under reference, steady state operating conditions, the detail to which the core power distribution can be known during much of the plant operating time, when only the ex-core neutron detectors are in active service, is quite limited and so this approach carries with it certain penalties.
Some drawbacks of using only either ex-core neutron detectors or a few strings of fixed in-core detectors for power distribution monitoring with only relatively infrequent periodic reinforcement by the in-core movable detector system, include:
a high reliance on necessarily conservative, analytically based estimates of the severity of possible power peaking in the interior of the core which translates immediately into operational constraints and, in extreme cases, into compulsory power derating of the unit, with severe economic penalties.
the lack of an on-line monitor for radial or three dimensional power distribution which necessarily introduces uncertainty into the estimates of the burnup accumulated by each fuel assembly in the core especially if there is extensive control rod insertion. Burnup estimation errors can lead to unexpected power peaking in reload fuel cycles. The consideration is more important if load follow is performed routinely since it leads to more control rod movement and deeper insertion of the rods into the core, than base load operation at high power (The sequence of changes in reactor power output which is required to accommodate demand driven changes in electrical output of an electrical generating plant is commonly referred to as load follow).
the movable in-core system is a complex mechanical one which is subject to wear induced failures or malfunctions of its components. While it can be called upon to perform many duties related to power distribution monitoring, such increased usage is at a risk of increased system unreliability and of sharply increased maintenance requirements.
In present practice, thermocouples are installed at or just above the outlet nozzles of a fraction of the fuel assemblies in most commercial pressurized water nuclear power reactors. Typical reactor cores generally consist of from approximately one hundred to more than two hundred assemblies and the thermocouples are usually installed at approximately one out of four fuel assembly locations.
In the past, little use has been made of the information available from the core-exit thermocouples. Typically, the on-line plant process computer periodically samples the thermocouple voltages, converts the electrical samples into digital values in convenient engineering units, .degree.F. or .degree.C., and displays the results in the form of a core coolant outlet temperature map generated on a line printer. On request, the plant computer also converts the core coolant outlet temperature values at the measured locations to equivalent F.sub..DELTA.H values, i.e. the relative enthalpy rises, at those locations and displays the results in map form on the line printer. In addition the plant computer also provides a crude measure of quadrant power tilting by comparing F.sub..DELTA.H values at symmetric thermocouple locations.
In the event that a plant operator observes an apparent core power distribution anomaly as indicated, for example, by quadrant power tilt from the ex-core power range neutron detectors or by dissimilar coolant loop temperature rise values, he may review recent thermocouple maps to identify changes in the readings of certain thermocouples which would confirm the existence, and identify the possible cause, of the power distribution anomaly. However, in practice, the information available from the core-exit thermocouple system is virtually never used as a primary diagnostic means and is not used in actually formulating estimates of the current core power distribution.
Thus, a need exists for a method and apparatus for monitoring and displaying the information available from the core-exit thermocouple system and for incorporating that information with the information that is derivable from the ex-core neutron detector system or from a few-string fixed in-core detector system in order to eliminate the drawbacks noted above that exist in current core monitoring systems that are not based on the availability of a large number of fixed in-core nuclear detectors. Satisfaction of that need will enhance both the safety-related and commercial aspects of PWR operation.